1. Field of the invention
The present invention concerns the fabrication of a double channel semiconductor laser.
2. Description of the prior art
This type of laser structure is internationally known by the acronym DCPBH which stands for "Double Channel Planar Buried Heterostructure". It may be fabricated from a base material in the form of indium phosphide and an active material further comprising gallium arsenide. It can then be used as a pump laser emitting a wavelength of 1 480 nm that can be used in long-haul fiber optic links including erbium-doped amplifying fibers.
A laser of this kind comprises the following elements which, in terms of their functions as stated hereinafter, are common to prior art lasers and a laser in accordance with the invention and which, in the case of a laser in accordance with the invention, are as shown in transverse cross-section in FIG. 1:
A (usually semiconductor) wafer 2 featuring internal crystal lattice continuity and having a thickness between two main surfaces 4, 8. The wafer defines a vertical direction Z which is that of its thickness and longitudinal X and transverse Y directions which are horizontal and mutually perpendicular. The two main surfaces are a lower surface 4 covered with a lower electrode 6 and an upper surface 8 covered with an upper electrode 10. The wafer is made up of layers in vertical succession from the lower surface to the upper surface. It has diversified areas extending longitudinally and juxtaposed transversely. One such area is a stripe area ZR in which the wafer comprises the following successive layers:
a lower injection layer 25 having a first type of conductivity,
an active layer 14 constituting a laser stripe 14A within the area ZR, and
an upper injection layer 16 having a second type of conductivity.
These layers constitute an active system. They are so fabricated that a polarization current connected via the electrodes 6, 10 injects vertically into the laser stripe 14A from the injection layer charge carriers of opposite types enabling the amplification of laser light within the stripe. The stripe has a refractive index which is higher than that of the surrounding layers 12, 25, 16, 26, 20, 22 to constitute a guide for this light. The injection layers extend transversely beyond the stripe area. The wafer 2 has two transverse sequences of areas each running from a respective side of the stripe area ZR. Each such succession comprises firstly a near lateral area ZC in which a lateral channel CL is substituted for the upper injection layer, the active layer and at least an upper part of the lower injection layer. This channel comprises a lower blocking layer 20 having the second type of conductivity on top of which is an upper blocking layer 22 having the first type of conductivity, so that the two blocking layers form between them a blocking junction JB for the polarization current constituting near current blocking means tending to confine the current within the stripe area ZR.
The transverse sequence of areas secondly comprises a far lateral area ZD in which an electrically resistive layer constitutes far current blocking means cooperating with the near current blocking means to confine the polarization current within the stripe area. The implementation of the far current blocking means is specific to the present invention and will be described in more detail later.
Still referring to features common to prior art lasers and a laser in accordance with the invention, the blocking layers 20, 22 extend into the far lateral area but not into the stripe area, this being achieved by the use of appropriate epitaxial growth techniques to grow layers. The blocking junction that they form blocks the current in the channels because they are thick enough here. On the other hand, the blocking is less than perfect in the far lateral layers beause they are thinner there.
This is why far current blocking means are required.
The upper injection layer 16 and these blocking layers have on top of them a thicker additional layer 26 having the second type of conductivity on top of which is a more strongly doped contact layer 28.
Two mirrors (not shown) are formed by the longitudinally remote surfaces of the semiconductor wafer to constitute a laser emitter.
A first prior art laser comprising these common features and arrangements is described in the JAPANESE JOURNAL OF APPLIED PHYSICS. vol. 25, No 6, June 1986, PART II, TOKYO, JP pages 435-436; S.L. SHI ET AL: "BH INGaAsP lasers with an LPE grown semi-insulating layer".
In this laser the energizing current is blocked in the near lateral area and in the far lateral area by three stacked layers grown epitaxially. Two of these layers form a blocking semiconductor junction. The third comprises modifying impurities (cobalt) imparting sufficient electrical resistivity to it for it to be semi-insulative.
This arrangement has the drawback that pinching of the layer deposited in this way may occur at the outer edges of the channels, that is to say at the boundary between the near and far lateral areas. Any such pinching prevents the required electrical confinement.
A second prior art laser comprising these common features and arrangements is described in European patent EP-A 161 016 (Philips).
In the second prior art laser the far current blocking means are formed, after a wafer is fabricated as previously described, by ionic bombardment localized to the far lateral area which induces therein crystalline defects increasing the resistivity of the base material. Protons are used for the ionic bombardment to achieve an adequate depth of penetration. The electrical resistivity created in the area in this way may initially be sufficiently high to enable correct operation of the laser. However, the crystalline defects which are the reason for this are progressively healed if the temperature of the wafer increases, which reduces the reliability and the range of uses of the laser.
An object of the present invention is to enable simple fabrication of a laser which is reliable even at high temperatures and which is easy to manipulate, even during fabrication.